The present invention relates to heat-shrinkable films, especially multilayer, heat-shrinkable films suitable for use in the packaging of products. The present invention is also directed to a packaging method using such films.
There is a need for multilayer heat-shrinkable films and articles of manufacture made therefrom, which have high impact strength, especially at elevated temperatures, high free shrink at 185xc2x0 F., high modulus, high gloss and package presentation, good sealability and seal strength, and stack/overlap sealing capability, and which can be easily oriented. This combination of features is not currently available. There is also a strong desire in the marketplace for thin films which also possess the above-described combination of features. The process of downgauging films without sacrificing performance attributes not only utilizes less polymeric material (which is better for the environment) but also lowers the cost to the end-user.
Recently it has been discovered that certain commercially-available bags can be sealed when stacked on top of one another, i.e., without sticking to one another. This non-sticking characteristic provides an advantage for packaging in a vacuum chamber, because the chamber, although typically having only one sealing means, has more than enough space therewithin for multiple bagged products which are to be sealed after evacuation of the atmosphere from the chamber. Thus, the non-sticking feature enables the evacuation and sealing of more than one bag at a time in a vacuum chamber, thereby increasing the production rate of the vacuum chamber packaging apparatus.
U.S. Pat. No. 5,336,549, to Nishimoto et al., discloses a heat-shrinkable film which can be made into bags. Apparently, users of this film, which is commercially available, have discovered that bags made from the film can be stacked on top of one another during sealing, without sticking to one another (i.e., the bags are xe2x80x9cstack-sealablexe2x80x9d). This enables the output of vacuum chamber packaging machinery to be, for example, doubled, if two bags are stacked on top of one another and simultaneously sealed.
The film disclosed in the ""549 patent has an outer layer of a polyester, and an intermediate layer of a polyamide having a melting point of higher than 160xc2x0 C. and lower than 210xc2x0 C. Although Nishimoto et al discloses a large group of polyamides for use in an inner layer, together with various polyesters for use in an outer layer, Nishimoto does not disclose the use of an inner layer comprising polyester.
We have discovered films which can provide a combination of desirable characteristics: high impact strength, especially at elevated temperatures, high free shrink at 185xc2x0 F., high modulus, high gloss and package presentation, good sealability and seal strength, and stack/overlap sealing capability. Moreover, in the process of making these films, the orientation step is not difficult. In addition, films in accordance with our invention can be made significantly thinner than prior art polyolefinic films and still possess comparable or superior performance attributes.
Moreover, we have discovered that at least the film of Example 1, below, which is in accordance with the present invention, exhibits one or more of the following advantages over heat-shrinkable films which are predominantly polyolefinic: a higher impact strength at both room temperature and elevated temperature, superior optics (e.g., gloss and haze), and, superior grease-resistance.
Furthermore, it has also been discovered that it is not difficult to orient a cast tape in the process of making the heat-shrinkable film in accordance with our invention. Orientation can be carried out to a high degree, versus prior.art films which comprise an inner polyamide layer having a high melting point. The ease of orientability and wider orientation window provided by the films of this invention also result in a stable orientation process. While the film of our invention can also provide high total free shrink, the process of orientation at a lower temperature can also enhance the free shrink of our multilayer film. Furthermore, the films of this invention can be made so that they are relatively free of optical defects (such as die-lines), versus films comprising a polyamide with a high melting point, i.e., greater than 160xc2x0 C. (e.g., polyamide 6, polyamide 66, polyamide 6/66, polyamide 6.6/6.10, polyamide 12, etc.). The use of an inner layer comprising polyester and an outer layer comprising a polyester is especially preferred in the films of this invention. The use of a polyester is also preferred in the films of this invention because it is significantly less expensive than a polyamide.
In the packaging of a relatively rigid product which is not distorted by forces produced by a shrinking film, it is generally desirable to provide a heat-shrinkable packaging film with as high a free-shrink as possible, in order to provide the xe2x80x9ctightestxe2x80x9d possible packaging over the product. In general, a tighter package provides a superior appearance, all other factors remaining the same. Our film has a relatively high free shrink, thereby enabling improved product appearance over a film having a lower free shrink.
As a first aspect, the present invention is directed to a heat-shrinkable multilayer film comprising: (A) a first layer, which is an outer layer, and which comprises polyolefin; (B) a second layer comprising at least one member selected from the group consisting of polyolefin, polystyrene, and polyurethane; (C) a third layer comprising at least one member selected from the group consisting of amorphous polyester and polyester having a melting point of from about 130xc2x0 C. to about 260xc2x0 C.; and (D) a fourth layer, which is an outer layer, the fourth layer comprising at least one member selected from the group consisting of polyester, polyamide and polyurethane.
Preferably, the film has a total free shrink, at 185xc2x0 F., of from about 40 to about 170 percent; more preferably, from about 50 to about 150 percent; more preferably, from about 60 to about 130 percent; more preferably, from about 65 to about 110 percent; more preferably, from about 70 to about 100 percent; and, more preferably, from about 75 to about 95 percent.
Preferably, the film has a thickness uniformity of at least 20 percent; more preferably, at least 30 percent; still more preferably, at least 40 percent; yet still more preferably, at least 50 percent; even yet still more preferably, at least 60 percent; still more preferably, at least 70 percent; still more preferably, at least 80 percent; and, still more preferably, at least 85 percent.
Preferably, the third layer comprises an amorphous polyester.
The fourth layer comprises at least one member selected from the group consisting of amorphous polyester and polyester having a melting point of from about 130xc2x0 C. to about 260xc2x0 C. If the fourth layer comprises a polyester having a melting point (i.e., a non-amorphous polyester), preferably this polyester has a melting point of from about 150xc2x0 C. to about 250xc2x0 C.; even more preferably, from about 170xc2x0 C. to about 250xc2x0 C.; still more preferably, from about 180xc2x0 C. to about 240xc2x0 C.; still more preferably, from about 190xc2x0 C. to about 240xc2x0 C.; still more preferably, from about 200xc2x0 C. to about 240xc2x0 C.; and yet still more preferably, from about 210 to about 235xc2x0 C. Preferably, the polyester in the fourth layer comprises from about 70 to about 95 mole percent terephthalate mer units; more preferably, from about 80 to about 95 mole percent terephthalate mer units; still more preferably, from about 85 to about 90 mole percent terephthalate mer units.
In yet another preferred embodiment, preferably the fourth layer comprises at least one member selected from the group consisting of amorphous polyamide and/or polyamide having a melting point of from about 130xc2x0 C. to about 260xc2x0 C. If the fourth layer comprises a polyamide having a melting point (i.e., a non-amorphous polyamide), preferably this polyamide has a melting point of from about 150xc2x0 C. to about 260xc2x0 C.; even more preferably, from about 170xc2x0 C. to about 250xc2x0 C.; still more preferably, from about 180xc2x0 C. to about 240xc2x0 C.; still more preferably, from about 190xc2x0 C. to about 240xc2x0 C.; still more preferably, from about 200xc2x0 C. to about 240xc2x0 C.; and yet still more preferably, from about 210 to about 235xc2x0 C.
Preferably, the film has a gloss of at least 50 percent, as measured against the fourth layer by ASTM D2457 (hereby incorporated in its entirety, by reference thereto); more preferably, the gloss is at least about 55 percent; more preferably, at least about 60 percent; more preferably, at least about 65 percent; more preferably, at least about 70 percent; and still more preferably, at least about 75%. Preferably, the film has a haze of no more than 10 percent, as measured by ASTM D 1003 (hereby incorporated, in its entirety, by reference thereto); more preferably, a haze of from about 0 to about 7 percent, still more preferably, from about 0 to about 5 percent.
Preferably, the film has a total thickness of from about 0.5 to about 10 mils, more preferably, from about 1 to about 5 mils; more preferably, from about 1.3 to about 4 mils; still more preferably, from about 1.5 to about 3.5 mils; yet still more preferably, from about 1.8 to about 2.5 mils.
Preferably, the film further comprises a fifth layer which serves as an O2-barrier layer, the fifth layer comprising at least one member selected from the group consisting of EVOH, PVDC, polyalkylene carbonate, polyamide, and polyethylene naphthalate. Preferably, the fifth layer is between the third layer and the fourth layer.
Preferably, the film further comprises a sixth layer which comprises at least one member selected from the group consisting of polyester and polyamide, wherein the sixth layer is between the fourth layer and the fifth layer. More preferably, the film further comprises a seventh layer which is a tie layer, the seventh layer being between the second layer and the third layer, and an eighth layer which is also a tie layer, the eighth layer being between the fourth layer and the sixth layer.
Preferably, the first layer comprises ethylene/alpha-olefin copolymer, the second layer comprises ethylene/vinyl acetate copolymer, the third layer comprises polyethylene terephthalate, the fourth layer comprises EVOH, and the fifth layer comprises polyethylene terephthalate. More preferably, the first layer comprises a blend of homogeneous ethylene/alpha-olefin copolymer and heterogeneous ethylene/alpha-olefin copolymer.
Preferably, the second layer is between the first layer and the third layer, the third layer is between the second layer and the fifth layer, and the fifth layer is between the third layer and the fourth layer.
Preferably, the first layer has a thickness of from about 1 to about 60 percent (more preferably, from about 10 to about 30 percent), based on total film thickness; the second layer has a thickness of from about 1 to about 50 percent (more preferably, from about 5 to about 25 percent), based on total film thickness; the third layer has a thickness of from about 5 to about 40 percent (more preferably, from about 10 to about 25 percent), based on total film thickness; the fourth layer has a thickness of from about 1 to about 40 percent (more preferably, from about 4 to about 20 percent), based on total film thickness; the fifth layer has a thickness of from about 1 to about 20 percent, based on total film thickness (more preferably, from about 5 to about 15 percent). Preferably, the film comprises a crosslinked polymer network.
Preferably, the film has an impact strength, as measured by ASTM 3763 (hereby incorporated by reference thereto, in its entirety), of at least about 60 Newtons (N); more preferably, from about 60 to about 500 N; yet more preferably, from about 70 to about 500 N; yet still more preferably, from about 80 to about 500 N; more preferably, from about 90 to about 500 N; more preferably, from about 100 to about 500 N; more preferably, from about 110 to about 500 N; more preferably, from about 120 to about 500 N.
Preferably, the film has an impact strength (peak load) at 190xc2x0 F. (88xc2x0 C.), as measured by ASTM 3763 conducted at 88xc2x0 C., of at least about 10 pounds (i.e., lbf or poundforce); more preferably, from about 10 to about 150 pounds; more preferably, from about 12 to about 100 pounds; more preferably, from about 14 to about 75 pounds; more preferably, from about 16 to about 60 pounds; more preferably, from about 18 to about 50 pounds; and more preferably, from about 20 to about 40 pounds.
As a second aspect, the present invention pertains to a bag made from the film according to the first aspect of the present invention. Preferably, the bag is made from a preferred film according to the first aspect of the present invention. Preferably, the bag is produced by sealing the first layer to itself, whereby the first layer is an inside bag layer and the fourth layer is an outside bag layer. Preferably, the bag is made from a preferred film according to the first aspect of the present invention. The bag can be an end-seal bag, a side-seal bag, an L-seal bag (i.e., sealed across the bottom and along one side, with an open top), or a pouch (i.e., sealed on three sides, with an open top).
As a third aspect, the present invention is directed to a process for packaging a product, comprising the steps of: (A) placing a first product into a flexible, heat-shrinkable bag which is in accordance with the second aspect of the present invention; (B) repeating the placing step with a second product and a second bag, whereby a second bagged product results, (C)stacking at least the first and second bagged products so that an excess bag length of each of the bagged products are within a sealing distance of a means for heat-sealing, and (D) heat-sealing the inside layer of first bag to itself in the region between the open end of the first bag and the first product, and the inside layer of the second bag to itself in the region between the open end of the second bag and the second product, so that the first product is completely sealed within the first bag and the second product is completely sealed with the second bag. The sealing is carried out at a temperature so that the resulting packaged products can be freely separated from one another without layer delamination. The bag has an open top so that prior to sealing, both the first bagged product and the second bagged product have excess bag length. During sealing of each bag, the first layer is sealed to itself, as the first layer is the inside layer in both the first bag and the second bag. Likewise, the fourth layer is the outside layer of the first bag and the second bag. The process can be carried out in a continuous, single, dual, or rotary chamber vacuum packaging machine. Preferably, from 2 to 5 bagged products are stacked on top of one another during heat-sealing. Preferably, the process utilizes a preferred bag in accordance with the present invention.